1. Field of the Invention
The present invention relates to a thin film merged magnetoresistive (MR) head which has aligned pole tips and to a method of manufacturing the head.
2. Description of the Related Art
In a magnetic disk drive, data is written and read by thin film magnetic transducers called "heads" which are supported over a surface of the disk while it is rotated at a high speed. The heads are supported by a thin cushion of air (an "air bearing") produced by the disk's high rotational speed.
Thin film magnetic write heads are desirable because they provide high areal density and thin film magnetic read heads are desirable because of their high resolution. Thin film magnetic heads are also easy to manufacture. With various thin film manufacturing techniques, the heads can be fabricated in batches on a ceramic substrate, and then diced into individual heads.
A thin film write head includes bottom and top pole pieces P1 and P2, respectively, that are formed from thin films ("layers") of magnetic material. The pole pieces have a pole tip height dimension commonly called "throat height". In a finished write head, throat height is measured between an air bearing surface ("ABS"), formed by polishing the tips of the pole pieces, and a zero throat height level ("zero throat level"), where the bottom pole piece P1 and the top pole piece P2 converge at the magnetic recording gap. A thin film magnetic write head also includes a pole tip region, which is located between the ABS and the zero throat level, and a back area, which extends back from the zero throat level to and including a back gap. Each pole piece has a pole tip portion in the pole tip region and a back portion in the back region. The pole pieces are connected together at the back gap.
The pole tips are extensions of the bottom and top pole pieces P1 and P2 of the write head. Each of the pole pieces P 1 and P2 transitions to a pole tip in the pole tip region. The pole tips are separated by a gap (G), which is a thin layer of insulation material. The pole tip of the top pole piece P2 is the last element to induce flux into a magnetic medium; therefore, its width is more important than the width of the pole tip on the bottom pole piece P1. However, as will be explained in detail hereinafter, it is important for the pole tips to have the same width so as to minimize flux leakage therebetween.
In order to increase the amount of data stored per unit of disk surface area ("areal density"), a write head must write more data in narrower tracks on the disk surface. Accordingly, areal density can be improved by decreasing the gap length between the pole tips. By decreasing the gap length, the bit density within a track is improved. The shortness of the gap length is limited by the decreasing flux intensity between the pole tips. Areal density can also be improved by increasing the number of data tracks which a write head can record on a disk; the related parametric expression is "tracks per inch" or "TPI". The TPI capability of a write head is increased by decreasing the head dimension which determines the width of a data track; typically this dimension is called the head "track width".
An MR read head employs a magnetoresistive (MR) element which changes resistance in response to magnetic flux density from a rotating magnetic disk. A sensing current, which is passed through the magnetoresistive element, varies proportionately to the change in resistance of the magnetoresistive element. The response of the magnetoresistive element is based on how well the resistance change of the magnetoresistive element follows the change in flux density sensed from the magnetic medium. In a disk drive, a differential preamplifier is connected to the magnetoresistive element for processing readback signals from the read head. The magnetoresistive element is a thin film layer which is sandwiched between bottom and top gap (insulation) layers G1 and G2 which, in turn, are sandwiched between bottom and top shield layers S1 and S2. The distance between the shield layers is called the read gap. The smaller the read gap, the greater the resolution of the MR read head.
A recent advance in technology has provided a merged MR head. A merged MR head employs an MR read head and a write head in combination. This is accomplished by using the top shield S2 of the MR head as the bottom pole P1 of the write head. A merged MR head has a high capability for either reading or writing. The merged MR head saves processing steps over constructing separate read and write heads because the second shield layer S2 of the MR read head also serves as the bottom pole P1 for the write head thereby eliminating a fabrication step. Another advantage of the merged MR head is that the elements of the read and write heads can be easily aligned on a single suspension system for reading immediately after writing.
However, present merged MR head structures generate significantly large side-fringing fields during recording. These fields are caused by flux leakage from the top pole P2 to the parts of the bottom pole P1 beyond the region defined by P2. The side-fringing fields limit the minimum track width achievable and therefore limit the upper reach of track density. Consequently, when a track written by the recording element of a merged MR head is read by the MR element, the "offtrack" performance of the MR element is poor. That is, when the MR element is moved laterally from the center of a track being read, it cannot move far before interference from the fields of the adjacent track begins to interfere with the fields of the track being read.
In an inductive head, the sidewalls of the bottom and top pole tips PT1 and PT2 are substantially vertically aligned and constrained to substantially equal widths by ion beam milling through the top and bottom pole pieces. However, because of shadowing caused by the top pole tip PT2 during this process, there is some outward taper to the bottom pole tip PT1. While the asymmetry of this taper results in some undesirable effects, the sidewalls of the pole tips are generally vertically aligned to prevent side-flinging beyond the edges of the gap between the pole tips.
The present methods for fabricating a merged MR head deposit a gap layer on top of the second shield layer S2 and then deposit the top pole tip PT2 on top of the gap layer. The pole tip PT2 can be defined either by photoresist frame plating or ion beam milling. The width of the pole tip PT2 is kept narrow, in the order of 5 .mu.m, so as to limit the width of written tracks. However, the second shield layer S2 of the MR read head is very wide, in the order of 50 .mu.m, in order to shield the MR element in the read head. The disparity between these widths results in a side-fringe flux field between the pole tip elements which extends laterally beyond the width of the top pole tip PT2. This is caused by the width of the second shield S2, which provides a large lateral channel for the flux lines from the top pole tip element PT2. It would be desirable for the second shield member S2, which comprises the bottom pole tip element PT1, to have sidewalls which are aligned with the sidewalls of the top pole tip element PT2. However, this is impossible since the second shield member S2 has to be wide in order to protect the MR element. This then would appear to prevent improvement in the off-track performance problem of the merged MR head.
One solution to the side-fringing problem of the merged MR head is to construct a narrow pole tip portion PT1b on top of the second shield layer S2, the S2 layer then serving as a wider, bottom pole tip element PT1a. Both of these pole tips are the pole tip portion of the bottom pole P1, with the pole tip layer PT1b forming a pedestal on the pole tip element PT1a. The gap layer is then formed on top of the pole tip layer PT1b and the pole tip element PT2 of the top pole piece P2 is formed on top of the gap layer. This pole tip arrangement can be constructed either of two ways: (1) frame plating each of the pole tips PT1b and PT2 using photoresist masking techniques or (2) masking the yoke area of the top pole P2 and ion beam milling through both of the pole tips PT2 and PT1b as well as the gap layer therebetween. In the frame plating process, it is extremely difficult to align the sidewalls of the pole tips PT2 and PT1b. This is because each of the pole tips is plated in a separate process, resulting in misalignment of the photoresist masks. In ion beam milling, redeposition of milling debris builds up on PT2 during the process, causing a shadowing of the pole tip layer PT1b therebelow. This shadowing, which is also encountered in constructing the inductive head discussed hereinabove, results in an outward taper configuration of the lower pole tip PT1b. Shadowing laterally extends the lower pole tip, and provides a magnetic path for side-fringing fields. Attempts have been made to remove the debris and to vertically align the sidewalls of these pole tips by directing the ion beam at an angle to the sidewalls rather than straight down. This will cut away some of the debris; however, the accumulated debris is too thick to allow obtaining vertical sidewalls by this process.